1. Field of the Invention
This invention relates to an electronic fuel injecting method and device for an internal combustion engine, and more particularly to improvements in an electronic fuel injecting method and device suitable for use in an internal combustion engine for a motor vehicle having a D-J type electronic fuel injection system, wherein a basic injection time is obtained in accordance with an intake pressure of the engine and an engine rotational speed, and, during transition, the basic injection time is corrected in accordance with the operating conditions of the engine so as to determine a fuel injection time.
2. Description of the Prior Art
The methods of supplying a mixture of a predetermined air-fuel ratio to combustion chambers of an internal combustion engine for a motor vehicle and the like include one using an electronic fuel injection system. According to this method, either a plurality of injectors, as many as the number of cylinders of the engine, or a single injector for the injection of fuel into the engine are provided, for example, on an intake manifold or a throttle body of the engine. The valve-opening time period of the injectors or injector is controlled in accordance with the operating conditions of the engine, so that a mixture of a predetermined air-fuel ratio can be supplied to the combustion chambers of the engine. This electronic fuel injection system is broadly divided into two systems including a so-called L-J type electronic fuel injection system wherein a basic injection time is obtained in accordance with an intake air flow rate of the engine and an engine rotational speed and a so-called D-J type electronic fuel injection system wherein a basic injection time is obtained in accordance with an intake pressure of the engine and an engine rotational speed.
The former can control the air-fuel ratio with high accuracy and is commonly used for the engines of motor vehicles to which is applied an exhaust gas purification system. However, in this L-J type electronic fuel injection system, the dynamic range of the intake air flow rate is so wide that the intake air flow rate at the time of high load is increased to about 50 times that at the time of idling, thereby presenting the following disadvantages. Namely, not only the accuracy is decreased when the intake air flow rate is converted into a digital signal, but also a bit length of the digital signal is lengthened when it is desired to improve the counting accuracy in a digital control circuit at the latter stage, whereby an expensive computer is required for the digital control circuit. Moreover, a measuring instrument having a construction with high accuracy such as an air flow meter or the like is required to measure the intake air flow rate, to thereby increase the installation cost.
On the other hand, the latter D-J type electronic fuel injection system has the features that the dynamic range of the intake pressure is so narrow that the variation value of the intake pressure is as low as two to three times. Not only is the operation in the digital control circuit at the latter stage facilitated, but also a pressure sensor for detecting the intake pressure is inexpensive. However, as compared with the L-J type electronic fuel injection system, the D-J type electronic fuel injection system has a low control accuracy of the air-fuel ratio, and particularly, has a low quality performance during acceleration because the fuel injection time is not increased until the intake pressure increases which lags the acceleration command so that the air-fuel ratio becomes lean temporarily. To obviate the disadvantages as described above, heretofore, an increased correction for acceleration has been provided in response to a pulse train fed from a comb-shaped sensor provided on a throttle valve. However, in order to improve the driverability, it is necessary to increase the increase correction to a considerable extent. In that case, the air-fuel ratio has become over-rich, and the value of carbon monoxide contained in the exhaust gas has increased to an unusually high extent, so that the air-fuel ratio could not be maintained within a predetermined range suitable for a three-way catalytic converter. This is also true where the fuel injection time is feedback controlled in response to an oxygen concentration sensor provided at the downstream side of the exhaust gas, because the oxygen concentration sensor is slow in response. In consequence, heretofore, it has been thought difficult to use the D-J type electronic fuel injection system in the engine for the motor vehicle, to which an exhaust gas purification system is applied, requiring air-fuel ratio control with high accuracy.
Furthermore, in the D-J type electronic fuel injection system, the fuel injection time is not decreased during deceleration unless the intake pressure decreases, whereby the air-fuel ratio becomes rich temporarily, thus degrading exhaust gas purification performance.